The present invention relates to a method for the production of rubber mixtures in a ram kneader or in an aggregate combination comprised of a ram kneader and a ram-less kneader disposed, preferably, underneath the ram kneader.
In connection with the production of rubber mixtures, one tries to achieve the lowest possible mixing temperature: as regards base mixtures or master batches, because of the distribution of fillers; as regards final mixtures, because of the reduced danger of premature vulcanization; and, as regards heat sensitive polymers such as, for example, natural rubber, to avoid damage of the polymer chains caused by the higher temperatures.
For decades, it has been an objective of the machine and process developments to achieve reduction of the mixing temperature via, for example, the development of kneaders with rotors which mesh with one another and the development of HESC rotors (high efficiency, super cooling rotors), via different positions of the cooling channels in the kneader pans or troughs, via the deployment of reduced revolutions per minute, and via many other measures.
These so-developed processes, however, have been directed to indirect cooling by which the pan or trough encircling the kneader, or the rotor, is cooled via water circulating in cooling channels and by which, accordingly, the heat occurring because of the mixing is conducted away. The water circulating in the cooling channels has no contact with the mix. If cracks occur in the kneader, the kneader must be shut off and repaired.
A basic disadvantage of the process of indirect cooling via circulating water in pans or troughs or in the rotors is that the heat conducting capability and the heat conducting capacities of the metal coatings in the pan or trough or, respectively, in the rotors, come into play, whereby it is known that, in particular in connection with highly wear resistant metal coatings, there exists only a poor heat conducting capability.
This leads to the result that the temperatures of the mixture and the metal masses have a counter-cyclic relationship—that is, in connection with the required cooling process, initially the metal masses of the kneader must be cooled during the mixing process before a meaningful cooling of the mixing components sets in. The production of rubber mixtures in one mixing step (“one step process”) is, in spite of many other advantages such as, for example, reduced space requirements, typically limited to small kneaders and/or plastic, slow vulcanizing mixtures and, as regards an adequate (online) cooling during the mix process, is often associated with long total mixing times.
In order to ensure the production of rubber mixtures having a sufficiently low mixing temperature, 80 to 90% of the total globally produced mixings, especially in large kneader tire mixings, have heretofore been produced via two step processes or greater than two step processes, whereby it has, as required, been necessary to discharge the mixings out of the kneader after each mixing step, form the intermediate mix product into a sheet via a rolling mill or a discharge extruder, cool off the sheet and, thereafter, perform one or more subsequent mixing processes. This manner of production is associated with high cost and time demands and is, for these reasons, uneconomical.
From EP 0 472 931 A1 and DE 37 02 833 A1, it is known to produce a rubber mixture in an aggregate combination comprised of a ram kneader and a ram-less kneader arranged under the ram kneader, whereby, especially in connection with the deployment of ram-less kneaders with meshing rotors, it is possible to cool the mixture product before the addition of further components thereto and, thus, to prepare in final form the mixture product during a build up cycle. Since the ram-less kneader is typically driven at low rates of rotation, it is possible to reduce the counter-cyclic temperature path between the kneader and the mixture, whereby the desired cooling of the mixture has already been effected at an earlier time and the cooling of the mixture is accelerated.
However, in connection as well with this process, especially in connection with the production of very hot running mixtures or in connection with mixtures which require only a very short mixing time, it is desirable to have a reinforcement or improvement of the cooling properties.
Any consideration of directly adding water to the rubber mixture for the cooling purposes runs into the reality that the rubber mixtures are almost exclusively composed of hydrophobic mixing components (rubber, paraffin softener, carbon black, paraffin wax, etc.) while water is hydrophilic.
EP 0 837 095 A1 discloses a method for the production of a silica-containing rubber mixture in which an improved distribution of the silica filler and good vulcanization properties can be achieved via mixing of the mixtures in accordance with completely predetermined temperature profiles. The coupling of the silica to the silane-coupling agent should be achieved at temperatures from 130 to 140° C. The temperature control can be effected via control of the rotational speed of the rotors or via the introduction of water directly into the mixture. The water is, in the latter instance, worked into the mixture. However, in this system, as well, in spite of the fact that a partially hydrophobic element is introduced into the mixture via the silica, there occurs an adhesion to mixtures having a basic lack of water, in that, for example, included water remnants remain in the mixture which, during subsequent working of the mixture such as, for example, during vulcanization, can lead to non-uniformities in the product.